ROME − Three international agencies urged Asians on Thursday to take special health precautions when raising or handling domestic ducks, which may carry the bird flu virus without showing any symptoms.

The U.N. Food and Agriculture Organization, the World Health Organization and the World Organization for Animal Health warned that seemingly healthy ducks might be infecting other poultry and possibly humans.

Concern is greatest in rural areas, where free-ranging ducks, chickens and wildlife often drink from the same water sources, the groups said in a statement. Ducks are also very common in the streets and market places of Southeast Asia and China.

People in affected areas should be careful not to drink water that has been in contact with ducks, the agencies said. Once the birds have been slaughtered, they should be scalded before their feathers are plucked.

The statement said that regional cooperation, reporting and controls were also crucial, and called for urgent research to establish how widespread the incidence of infection in ducks without symptoms has become. The agencies also urged research on the effectiveness of current vaccines on duck populations.

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Unlike chickens, which have been the main cause of concern, ducks generally carry the virus without being affected themselves, said Dr. Juan Lubroth with the animal health service of Rome-based FAO.

"We have found out that, despite not showing any signs, (ducks) contribute to the spread of the disease through secretions that contaminate the environment," such as sources of drinking water, he said in a telephone interview.

A new laboratory study has showed that healthy-looking ducks can spread just as much of the virus as chickens that are visibly ill, the agencies said.

The H5N1 strain of bird flu appeared throughout Asia early this year, ravaging poultry farms and sparking a region-wide health scare. Authorities culled tens of millions of birds in an attempt to thwart the spread of the disease, but it resurfaced in July. Bird flu has killed 20 people in Vietnam and 12 in Thailand.

Recent findings show that the virus has become more virulent in chickens and mice and can be carried by mammals, including certain cats and tigers, that were not previously considered at risk, the organizations said.